DEFINITION: The elderly population is the number of inhabitants of a given region aged 65 or older. The population can be either the average annual population or the population at a specific date during the year considered. The average population during a calendar year is generally calculated as the arithmetic mean of the population on 1 January of two consecutive years (it is also referred to as the mean population).The geographic concentration index offers an accurate picture of the spatial distribution of elderly population, as it takes into account the area of each region and reveals large international differences in the degree of geographic concentration of elderly people. The geographic concentration index compares the economic weight and the geographic weight over all regions in a given country and is constructed to account for both within- and between-country differences in the size of all regions. The index lies between 0 (no concentration) and 100 (maximum concentration) and is suitable for international comparisons of geographic concentration.